HIGH-STRENGTH POLYOLEFIN-COATED STEEL MATERIAL

Abstract: A polyolefin-coated steel material, in particular, one excellent in the resistances to impact, flaw and abrasion due to drifts of sand and so forth, and usable in steel structures. The invention material comprises an undercoated steel material and, formed on the surface thereof, a modified polyolefin resin layer and a polyolefin resin layer, and is characterized in that the polyolefin resin layer is further covered with a fiber cloth laminated with a modified polyolefin resin.

Description: The present invention relates to a polyolefin-covered steel product and particularly to a polyolefin-covered steel product for use in steel structures, having excellent impact resistance, flaw resistance, and resistance to abrasion by littoral drift or the like. Polyolefin-covered steel products, such as polyolefin-covered metal pipes and polyolefin-covered steel sheet piles, have become used widely as steel pipes for piping, heavy-duty anticorrosive steel pipe piles in the sea, and heavy-duty anticorrosive steel sheet piles, by virtue of excellent anticorrosive properties. The polyolefin-covered steel products, however, are likely to suffer dents during handling (such as installation of piping on site; flaws on the surface of the covering caused by soil, pebbles, and the like during driving of polyolefin-covered steel pipe and sheet piles; and flaws in the covering during execution of works of ripraps. Thus there is a requirement for the covering layer to have further improved mechanical strength and surface hardness. In order to solve these problems Japanese Unexamined Patent Publication (Kokai) No. 63-27248 proposes a polyolefin-covered steel product comprising a steel product which has been subjected to surface treatment, a modified polyolefin resin layer and a polyolefin resin layer laminated on the treated surface, and, laminated thereon, a fiber reinforced modified polyolefin resin layer comprising glass fiber or the like dispersed in a silane modified polyolefin resin, for the purpose of improving the impact and flaw resistance of the product. Even the latter product having improved impact and flaw resistance has the drawback that the covering, when exposed in a sandy coastal region rich in littoral drift, is markedly abraded, markedly impairing durability. Specifically, an enhancement in surface hardness and strength of the material to a relatively high value as in the case of the conventional covered steel products is effective in preventing flaws by the action of large instantaneous energy, such as flaws caused by collision and dents. However, the conventional covered steel products have the drawback that, when they are exposed to littoral drift or the like, which abrades their surface over a long period, they are gradually abraded because the covering is hard and fragile. Therefore, in the prior art, it was difficult to provide a polyolefin-covered steel product excellent in resistance to flaws caused by collision or dents, as well as in resistance to abrasion by littoral drift and the like. An object of the present invention is to provide a polyolefin-covered steel product, for use in steel structures, having excellent impact resistance, flaw resistance and resistance to abrasion by littoral drift The present invention provides a polyolefin-covered steel product having impact resistance, flaw resistance, and abrasion resistance, for use in a steel structure, comprising a steel product which has been subjected to surface treatment as a pretreatment, and having laminated onto the treated surface a modified polyolefin resin layer and a polyolefin resin layer, characterized in that the surface of said polyolefin resin layer has a covering of a fibre cloth, to which a modified polyolefin resin layer has been laminated, said covering having said fibre cloth innermost. The covering of fiber cloth to which a modified polyolefin resin has been laminated can offer advantage in that, for impact resistance, the fiber cloth can prevent through flaws; the fiber cloth having a higher hardness than the polyolefin resin layer as an anticorrosive layer can prevent flaws during handling; and the fiber cloth, which is hard but not fragile, has resistance also to abrasion by littoral drift over a long period. The above technique can thus provide a polyolefin-covered steel product excellent in impact and flaw resistance, and resistance to abrasion by littoral drift. Fig.1 of the accompanying drawings shows a polyolefin-covered steel product of the present invention having excellent impact and flaw resistance and resistance to abrasion by littoral drift or the like, comprising a steel product 1, which has been subjected to surface treatment as a pretreatment, and laminated onto the treated surface, a modified polyolefin resin layer 2 and a polyolefin resin layer 3, characterized in that the surface of the polyolefin resin layer 3 has a covering of a fiber cloth 4 to which a modified polyolefin resin layer has been laminated, the latter resin layer being outermost. The modified polyolefin resin of the covering may contain antioxidant and a colour pigment comprising an organic pigment or an inorganic pigment. The fibre cloth of the covering may be coloured. The surface treatment used according to the invention may comprise a chromate treatment to produce a chromate coating at a total Cr coverage of 10 to 1500 mg/m 2 , and optionally also an epoxy primer treatment to produce a primer coating having a thickness of 5 to 300 µm. The modified polyolefin resin layer preferably has a thickness of from 10 µm to 3 mm, the polyolefin resin layer a thickness of from 1 to 10 mm, and the modified polyolefin resin layer laminated onto said fiber cloth a thickness of from 5 µm to 10 mm. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a partially vertical sectional view of a high-strength polyolefin-covered steel product according to the present invention; and Fig. 2 is a schematic diagram showing an embodiment of a process for producing a high-strength polyolefin-covered steel pipe as an embodiment of the high-strength polyolefin-covered steel product according to the present invention. Fig. 1 is a partially sectional view of a high-strength polyolefin-covered steel product according to the present invention having a construction according to the present invention. In the drawing, 1 denotes a surface treated steel product , from which oils, scales, and the like have been removed by pickling, blasting, or the like, or a steel product of the same type as described above except that it has beep further subjected to conversion treatment, such as treatment with a chromate, an organic primer, a silane coupling agent, or the like, numeral 2 designates a modified polyolefin resin, numeral 3 designates a polyolefin resin, and numeral 4 designates a fiber cloth with a modified polyolefin resin laminated thereonto. Steel products usable in the present invention include steel sheets and plates, such as cold-rolled steel sheets, hot-rolled steel sheets, and thick steel plates, shape steels, such as H sections, I sections, and L sections, steel sheet piles, bar steels, steel wires, cast iron pipes, steel pipes, and steel pipe piles. These steel sheet and plates, shape steels, steel sheet piles, bar steels, steel wires, steel pipes, and steel pipe piles may have a surface plated with zinc, nickel, chromium, aluminum, zinc-aluminum, zinc-nickel, zinc-nickel-cobalt, and the like. In the present invention, the steel product which has been subjected to surface treatment is a steel product having a surface from which oils, scales, and the like have been removed by degreasing, pickling, sand blasting, grit blasting, shot blasting, or the like. Further, the surface of the steel product, from which oils, scales, and the like have been removed, may be subjected to conversion treatment involving one, or a combination of two or more, of the conventional treatment with a chromate, treatment with an organic primer, treatment with a silane coupling agent, and the like. The coverage of the chromate treatment is preferably in the range of from 10 to 1500 mg/m 2 in terms of the coverage of total Cr. When the coverage of Cr is less than 10 mg/m 2 , the adhesion between the covering material and the steel product is so low that, during an impact test, the covering is cracked and, at the same time, the covering material peels off. When the coverage of Cr is larger than 1500 mg/m 2 , the chromate treatment agent layer becomes so fragile that, when an impact is propagated through the surface of the covering material, the chromate treatment layer is broken, remarkably lowering the impact resistance. The thickness of the organic primer layer is preferably in the range of from 5 to 300 µm. When the thickness of the primer layer is less than 5 µm, the adhesion between the covering material and the steel product is so low that, during an impact test, the covering is cracked and, at the same time, the covering material is peeled off. On the other hand, when the thickness of the primer layer exceeds 300 µm, the strain accompanying curing in the primer coating becomes so large that the primer coating is cracked during an impact test and, at the same time, peeled off. If the above surface treatment is not carried out, the adhesion between the steel product and the modified polyolefin resin layer becomes so low that peeling occurs at the interface of the steel product and the modified polyolefin resin layer when the covered steel product is exposed to an environment wherein the covered steel product comes in contact with water for a long period of time, unfavorably lowering the long-term corrosion resistance of the steel product. The modified polyolefin resin and the polyolefin resin used in the present invention will now be described. The modified polyolefin resin used in the present invention refers to a polyolefin modified with an unsaturated carboxylic acid, such as maleic acid, acrylic acid, or methacrylic acid, or an anhydrides thereof, the above modified polyolefin properly diluted with a polyolefin, and a silane-modified polyolefin, produced by modifying a polyolefin with an unsaturated silane compound, such as vinyltrimethoxysilane, vinyltriethorysilane, vinylmethyldiethoxysilane, or vinylphenyldimethoxysilane, properly diluted with a polyolefin. The thickness of the modified polyolefin covering layer is in the range of from 10 µm to 3 mm, preferably in the range of from 50 to 300 µm. When the layer thickness is less than 10 µm, the adhesive force is insufficient for the polyolefin-covered steel product to maintain its corrosion resistance for a long period of time, causing cracking and peeling during an impact test. On the other hand, a layer thickness exceeding 3 mm is effective from the viewpoint of impact strength. However, it is unfavorable from the viewpoint of economy. The polyolefin resin used in the present invention is a blend of a low-density polyethylene, a medium-density polyethylene, a high-density polyethylene, or a straight-chain low-density polyethylene having a density of 0.92 to 0.96 g/cm 3 with at least one of a polypropylene and an ethylene-propylene block or random copolymer having a density of 0.87 to 0.91 g/cm 3 , and an ethylene-butene-1 block or random copolymer having a density of 0.89 to 0.94 g/cm 3 and the like. The covering layer thickness of the polyolefin resin is in the range of from 1 to 10 mm, preferably 2 to 4 mm. When the covering layer thickness of the polyolefin is less than 1.0 mm, cracking unfavorably occurs during an impact test. On the other hand, when the covering layer thickness of the polyolefin resin is larger than 10.0 mm, the impact strength increases with an increase in the covering layer thickness. This, however, is unfavorable from the viewpoint of economy. The thickness of the polyolefin resin layer 3 may be determined by taking into consideration the external load, impact force, and economy in an actual environment. Further, the polyolefin resin layer 3 may contain carbon black or may be made black to impart weather resistance enough to be used in the outdoors for a long period of time. Furthermore, when a colored polyolefin resin is used for reasons of harmonization with environment and the like, various organic pigments and inorganic pigments may be added alone or in the form of a mixture of two or more for coloring purposes. Furthermore, any antioxidant and the like may be added from the viewpoint of improving the weather resistance. The adoption of the above techniques does not change the subject matter of the present invention. The fiber cloth 4, with a modified polyolefin resin being laminated thereto, used in the present invention is such a laminated fiber cloth as produced by providing any one of the above modified polyolefin resins, more specifically a modified polyolefin resin identical to the modified polyolefin resin used for adhering between a steel product and a polyolefin resin as an underlying layer covering the modified polyolefin resin-laminated fiber cloth 4, previously melting the modified polyolefin resin, extruding the melted modified polyolefin resin into a sheet, laminating the sheet to a fiber cloth, and strongly heat-sealing the laminate. In this case, in order to improve the impregnation of the fiber cloth with the modified polyolefin resin, the modified polyolefin resin to be laminated on the fiber cloth may be those which have a higher melt index than the modified polyolefin resin used for adhering the polyolefin resin to the steel product. Further, in order to enhance the adhesion between the fiber cloth and the modified polyolefin resin, the fiber cloth may be previously coated with the conventional surface treatment. The thickness of the modified polyolefin resin in the laminate is in the range of from 5 µm to 10 mm. However, it is preferably about 3.0 mm from the viewpoint of economy because the modified polyolefin resin is expensive. When the thickness of the modified polyolefin resin layer in the laminate is smaller than 5 µm, it is unfavorably difficult to stably extrude the modified polyolefin resin into a sheet. Further, when the thickness of the modified polyolefin resin layer is smaller than 5 µm, the fiber cloth cannot be sufficiently evenly laminated thereto. In this case, although the occurrence of cracking and peeling during an impact test can be prevented, the surface roughness is so large that the covering is likely to be damaged during a flaw resistance test. Carbon black is generally added to the modified polyolefin resin layer to be impregnated into the fiber cloth to turn the modified polyolefin resin black, thereby imparting weather resistance sufficient to withstand the use of the covered steel product in the outdoors for a long period of time. If a colored modified polyolefin resin is used for reasons of harmonization with a scene and the like, various organic pigments and inorganic pigments may be added alone or in the form of a mixture of two or more in order to color the modified polyolefin resin. This does not change the subject matter of the present invention. Moreover, in order to further improve the weather resistance of the colored polyolefin resin, it is also possible to add any antioxidant and the like to the modified polyolefin resin. The fiber cloth 4, to which the modified polyolefin resin is to be laminated, may be any fiber cloth independently of inorganic fiber cloths (including metallic fiber cloths) and organic fiber cloths, including glass fiber cloths, such as glass mat MC450S-020SE (RTM), glass cloth WF300-10N (RTM), and glass roving RS240PE-525 (RTM) all made by Nitto Boseki Co., Ltd.., a metallic fiber cloth, a vinyl cloth, asbestos, a carbon fiber mat, or a nonwoven fabric made of polyester, acryl, polypropylene, rayon, or the like. The above fiber cloth may previously have any color, which does not change the subject matter of the present invention. In order to reduce through flaws caused by impact, that is, in order to further improve the impact strength, it is preferred for the size of the stitch of the fiber cloth to be smaller. However, the density of the stitch is preferably relatively low from the viewpoint of sufficiently fusing the upper and lower modified polyolefin resins, which sandwich the fiber cloth therebetween, to each other. Therefore, when a covered steel product having a very high impact strength is desired, it is possible to adopt a method wherein a fiber cloth having a very fine stitch is used and a conventional surface treatment or the like is previously coated onto the fiber cloth to further improve the adhesion between the fiber cloth and the upper and lower modified polyolefin resins. The smaller the fiber diameter of the fiber cloth, the better the appearance of the covering of the polyolefin-covered steel product obtained. Further, the thickness of the fiber cloth is preferably much smaller than the thickness of the modified polyolefin resin sheet to be laminated. When an inorganic fiber cloth, particularly a metallic fiber cloth is sandwiched between modified polyolefin resins, a deterioration in the modified polyolefin resin by oxidation due to contact between the modified polyolefin resin and the metal is expected. Therefore, in this case, it is preferred to previously add, besides an antioxidant usually added to the modified polyolefin resin, other antioxidants in a necessary amount. Therefore, when the type of the fiber cloth with the modified polyolefin resin being laminated thereto is determined, the layer thickness of the modified polyolefin resin, the type of the fiber cloth, and the like can be determined by sufficiently taking into consideration the external load and impact force in environment, under which the polyolefin-covered steel product is used, profitability, and the like. The process for producing the high-strength polyolefin-covered steel product according to the present invention will now be described by taking a high-strength polyolefin-covered steel pipe as an example. The polyolefin-covered steel pipe can be produced, for example, by a production process shown in Fig. 2. Specifically, a chromate is coated on the surface of a steel pipe 1, from which scales and the like have been removed, by means of a chromate treatment coater 6, and the resultant coating is baked by means of a heater 7. An epoxy primer is then coated thereon by means of an epoxy primer coater 8, and the resultant coating is heat-cured by means of a post-heating device 9. Then, a modified polyolefin resin 2 is coated thereon by means of a modified polyolefin resin coater 10, and a polyolefin resin 3 is extrusion-coated thereon by means of a T die 11. Thereafter, the coated steel product is covered with fibre cloth 4 , to which a modified polyolefin resin has been laminated, by means of a device 5 for covering a fiber cloth with a modified polyolefin resin being laminated thereto, and sufficient pressure is applied to the resultant assembly by means of a pressure roll 13 so that the modified polyolefin resin and the polyolefin resin in the laminate can be strongly bonded to each other, followed by cooling with a cooler 12. Thus, the high-strength polyolefin-covered steel pipe according to the present invention is prepared. In Fig. 2, a method is used wherein a powder of a polyolefin resin is coated by electrostatic coating using a modified polyolefin resin coater 10. It is also possible to use conventional methods, such as a method wherein the modified polyolefin resin extrusion-coated by means of a T die or a ring die and a method wherein the modified polyolefin resin and the polyolefin resin are extrusion-coated in the form of two layers integral with each other by means of a T die. The present invention will now be described in more detail with reference to the following examples, though it is not limited to these examples only. EXAMPLES Production examples of a steel pipe covered with a high-strength, low-density polyethylene according to the present invention will now be described. A steel pipe (200A × 5500 mm in length × 5.8 mm in thickness) was grit-blasted, and a chromate treatment was blade-coated onto the surface thereof at a total Cr coverage of 500 mg/m 2 . The resultant coating was heated at 190°C for 3 min to carry out baking, an epoxy primer was coated by means of a spray coater to a thickness of 50 µm, and the resultant coating was then cured. Subsequently, a modified polyethylene resin was coated by electrostatic coating to a thickness of 200 µm, and a low-density polyethylene resin was extrusion-coated by means of a T-die to a thickness of 3.0 mm. Then, the resultant coating was covered with a fiber cloth (thickness: 2.0 mm), to which a modified polyolefin resin has been previously laminated, and the fiber cloth with the modified polyolefin resin layer as the uppermost layer being laminated thereonto was strongly bonded to the polyolefin resin layer as a corrosion-resistant layer. The assembly was then cooled to produce a high-strength, low-density polyethylene-covered steel pipe according to the present invention. For Examples 1 to 7, the above steel pipes covered with a low-density polyethylene were produced with various types of fiber. For Examples 8 to 10, steel pipes covered with a high-strength polyolefin were produced in the same manner as described above, except that a high-density polyethylene, a straight-chain low-density polyethylene, and polypropylene were used instead of the low-density polyethylene resin. Covering material Fiber cloth Modified polyolefin resin Layer thickness (mm) Surface treatment of fiber cloth Ex. of Invention 1 Glass cloth Maleic anhydride-modified PE 2.0 Not done 2 Glass mat Maleic anhydride-modified PE 2.0 Not done 3 Glass roving Maleic anhydride-modified PE 2.0 Not done 4 Metallic fiber cloth Maleic anhydride-modified PE 2.0 Not done 5 Vinyl cloth Maleic anhydride-modified PE 2.0 Not done 6 Asbestos Maleic anhydride-modified PE 2.0 Not done 7 Polyester bonded fabric Maleic anhydride-modified PE 2.0 Not done 8 Glass cloth Maleic anhydride-modified PE 2.0 Not done 9 Glass cloth Maleic anhydride-modified PE 2.0 Not done 10 Glass cloth Maleic anhydride-modified PP 2.0 Not done Results of abrasion test Results of impact test Flaw resistance test Depth of abrasion (mm) Pinhole or crack Peeling Damage to covering Ex. of Invention 1 <0.01 None occurred None occurred None occurred 2 <0.01 None occurred None occurred None occurred 3 <0.01 None occurred None occurred None occurred 4 <0.01 None occurred None occurred None occurred 5 <0.01 None occurred None occurred None occurred 6 <0.01 None occurred None occurred None occurred 7 <0.01 None occurred None occurred None occurred 8 <0.01 None occurred None occurred None occurred 9 <0.01 None occurred None occurred None occurred 10 <0.01 None occurred None occurred None occurred It is noted that the left side of Table 2 is placed next to the right side of Table 1, and the left side of Table 3 is placed next to the right side of Table 2 to constitute one table. For Examples 11 to 19, steel pipes covered with a high-strength color polyethylene were produced in the same manner as described above, except that the steel pipe was covered with a color polyethylene, a modified color polyethylene, or a colored fiber cloth. For Examples 20 to 29, steel pipes covered with a high-strength polyethylene were produced in the same manner as described above, except that the covering thickness was varied. For Example 30, a steel pipe covered with a high-strength polyethylene was produced in the same manner as described above, except that the fiber cloth, to which a modified polyolefin resin was to be laminated, had been previously surface-treated with a silane coupling agent. The resultant polyolefin-covered steel pipes were tested for impact resistance, flaw resistance, and resistance to abrasion with littoral drift. The littoral drift abrasion test was carried out by actually exposing a sample at a coastline for two years and then measuring the abrasion loss of the covering material. The impact resistance test was carried out by dropping a bell having a size of 5/8 in. and a weight of 10 kg from a height of 1m onto a sample to examine whether or not cracking and/or peeling of the covering occurred. The flaw resistance test was carried out by raising a sample to a height of 10m and lowering the sample using a wire sling 500 times to examine whether or not damage to the covering occurred. The results of the above tests are given in the above Tables 1 to 3 and the following Tables 4 to 12. The left side of Table 5 is placed on the right side of Table 4, and the left side of Table 6 is placed on the right side of Table 5 to constitute one table. The left side of Table 8 is placed on the right side of Table 7, and the left side of Table 9 is placed on the right side of Table 8 to constitute one table. Further, the left side of Table 11 is placed on the right side of Table 10, and the left side of Table 12 is placed on the right side of Table 11 to constitute one table. Covering material Fiber cloth Modified polyolefin resin Layer thickness (mm) Surface treatment of fiber cloth Ex. of Invention 11 Glass cloth Maleic anhydride-modified PE 2.0 Not done 12 Glass cloth Maleic anhydride-modified PE 2.0 Not done 13 Glass cloth Maleic anhydride-modified PE 2.0 Not done 14 Glass cloth Maleic anhydride-modified PE 2.0 Not done 15 Glass cloth Maleic anhydride-modified PE colored with organic pigment 2.0 Not done Results of abrasion test Results of impact test Flaw resistance test Depth of abrasion (mm) Pinhole or crack Peeling Damage to covering Ex. of Invention 11 <0.01 None occurred None occurred None occurred 12 <0.01 None occurred None occurred None occurred 13 <0.01 None occurred None occurred None occurred 14 <0.01 None occurred None occurred None occurred 15 <0.01 None occurred None occurred None occurred Covering material Fiber cloth Modified polyolefin resin Layer thickness (mm) Surface treatment of fiber cloth Ex. of Invention 16 Glass cloth Maleic anhydride-modified PE colored with inorganic pigment 2.0 Not done 17 Glass cloth Maleic anhydride-modified PE colored with organic pigment 2.0 Not done 18 Glass cloth Maleic anhydride-modified PE colored with inorganic pigment 2.0 Not done 19 Colored glass cloth Maleic anhydride-modified PE 2.0 Not done 20 Glass cloth Maleic anhydride-modified PE 2.0 Not done 21 Glass cloth Maleic anhydride-modified PE 2.0 Not done 22 Glass cloth Maleic anhydride-modified PE 2.0 Not done 23 Glass cloth Maleic anhydride-modified PE 2.0 Not done Results of abrasion test Results of impact test Flaw resistance test Depth of abrasion (mm) Pinhole or crack Peeling Damage to covering Ex. of Invention 16 <0.01 None occurred None occurred None occurred 17 <0.01 None occurred None occurred None occurred 18 <0.01 None occurred None occurred None occurred 19 <0.01 None occurred None occurred None occurred 20 <0.01 None occurred None occurred None occurred 21 <0.01 None occurred None occurred None occurred 22 <0.01 None occurred None occurred None occurred 23 <0.01 None occurred None occurred None occurred Covering material Fiber cloth Modified polyolefin resin Layer thickness (mm) Surface treatment of fiber cloth Ex. of Invention 24 Glass cloth Maleic anhydride-modified PE 2.0 Not done 25 Glass cloth Maleic anhydride-modified PE 2.0 Not done 26 Glass cloth Maleic anhydride-modified PE 2.0 Not done 27 Glass cloth Maleic anhydride-modified PE 2.0 Not done 28 Glass cloth Maleic anhydride-modified PE 0.5 Not done 29 Glass cloth Maleic anhydride-modified PE 10.0 Not done 30 Glass cloth Maleic anhydride-modified PE 2.0 Done Results of abrasion test Results of impact test Flaw resistance test Depth of abrasion (mm) Pinhole or crack Peeling Damage to covering Ex. of Invention 24 <0.01 None occurred None occurred None occurred 25 <0.01 None occurred None occurred None occurred 26 <0.01 None occurred None occurred None occurred 27 <0.01 None occurred None occurred None occurred 28 <0.01 None occurred None occurred None occurred 29 <0.01 None occurred None occurred None occurred 30 <0.01 None occurred None occurred None occurred The above procedure was repeated to produce a polyethylene-covered steel pipe wherein the polyethylene covering is not covered with a fiber cloth with a modified polyolefin resin being laminated thereonto (Comparative Example 1), a steel pipe wherein the uppermost layer was covered with a glass fiber reinforced polyethylene resin comprising short fibers dispersed in a polyethylene resin (Comparative Example 2), a steel pipe covered with a fiber cloth to which no modified polyolefin resin had been laminated (Comparative Example 3), and polyethylene-covered steel pipes of the same types as described above, except that the thickness of the covering material was varied (Comparative Examples 4 to 12). The covered steel pipes thus obtained were tested for resistance to abrasion by littoral drift, for impact resistance, and for flaw resistance. The results are also given in Tables 13 to 18. The left side of Table 14 is placed on the right side of Table 13, and the left side of Table 15 is placed on the right side of Table 14 to constitute one table. Further, the left side of Table 17 was placed on the right side of Table 16, and the left side of Table 18 is placed on the right side of Table 17 to constitute one table. Covering material Fiber cloth Modified polyolefin resin Layer thickness (mm) Surface treatment of fiber cloth Comp. Ex. 1 None Maleic anhydride-modified PE 2.0 Not done 2 Glass cloth Maleic anhydride-modified PE 2.0 Not done 3 Glass cloth Maleic anhydride-modified PE 2.0 Not done 4 Glass cloth Maleic anhydride-modified PE 2.0 Not done 5 Glass cloth Maleic anhydride-modified PE 2.0 Not done 6 Glass cloth Maleic anhydride-modified PE 2.0 Not done 7 Glass cloth Maleic anhydride-modified PE 2.0 Not done 8 Glass cloth Maleic anhydride-modified PE 2.0 Not done Results of abrasion test Results of impact test Flaw resistance test Depth of abrasion (mm) Pinhole or crack Peeling Damage to covering Comp. Ex. 1 0.05 Occurred None occurred Occurred 2 2.00 None occurred Occurred None occurred 3 <0.01 Occurred Occurred None occurred 4 <0.01 None occurred Occurred None occurred 5 <0.01 Occurred Occurred None occurred 6 <0.01 Occurred Occurred None occurred 7 <0.01 Occurred Occurred None occurred 8 <0.01 None occurred None occurred None occurred Covering material Fiber cloth Modified polyolefin resin Layer thickness (mm) Surface treatment of fiber cloth Comp. Ex. 9 Glass cloth Maleic anhydride-modified PE 2.0 Not done 10 Glass cloth Maleic anhydride-modified PE 2.0 Not done 11 Glass cloth Maleic anhydride-modified PE 0.004 Not done 12 Glass cloth Maleic anhydride-modified PE 10.5 Not done Results of abrasion test Results of impact test Flaw resistance test Depth of abrasion (mm) Pinhole or crack Peeling Damage to covering Comp. Ex. 9 <0.01 Occurred Occurred None occurred 10 <0.01 None occurred None occurred None occurred 11 0.5 None occurred None occurred Occurred 12 <0.01 None occurred None occurred None occurred As is apparent from Tables 1 to 18, in a polyolefin-covered steel pipe comprising a steel product which has been subjected to surface treatment, and having laminated onto the treated surface, a modified polyolefin resin layer 2 and a polyolefin resin layer 3, when the surface of the polyolefin resin layer 3 is covered with a fiber cloth 4 to which a modified polyolefin resin has been laminated, the resultant high-strength covered steel pipe exhibits very good results, unattainable in the prior art, in an impact test and a flaw resistance test, as well as in a test on resistance to abrasion by littoral drift. [Industrial Applicability] As is apparent from the above examples, the polyolefin-covered steel product according to the present invention is much superior to conventional polyolefin-covered steel products in impact resistance and flaw resistance, as well as in resistance to abrasion by littoral drift, by virtue of the provision of a fiber cloth 4, to which a modified polyolefin resin has been laminated, on a polyolefin covering layer 3 as an anticorrosive layer, so that the present invention has a marked advantage over the polyolefin-covered steel products of the prior art.

Claims: A polyolefin-covered steel product having impact resistance, flaw resistance, and abrasion resistance, for use in a steel structure, comprising a steel product which has been subjected to surface treatment as a pretreatment, and having laminated onto the treated surface a modified polyolefin resin layer and a polyolefin resin layer, characterized in that the surface of said polyolefin resin layer has a covering of a fibre cloth to which a modified polyolefin resin layer has been laminated, said covering having said fibre cloth innermost. A product according to claim 1 wherein the modified polyolefin resin of said covering contains antioxidant and a colour pigment comprising an organic pigment or an inorganic pigment. A product according to claim 1 wherein the fibre cloth of said covering is coloured. A product according to any one of claims 1 to 3 wherein said surface treatment comprises a chromate treatment to produce a chromate coating at a total Cr coverage of 10 to 1500 mg/m 2 . A product according to any one of claims 1 to 3 wherein said surface treatment comprises a chromate treatment to produce a chromate coating at a total Cr coverage of 10 to 1500 mg/m 2 and an epoxy primer treatment to produce a primer coating having a thickness of 5 to 300 µm. A product according to any preceding claim wherein said modified polyolefin resin layer has a thickness of from 10 µm to 3 mm, said polyolefin resin layer has a thickness of from 1 to 10 mm, and said modified polyolefin resin layer laminated onto said fibre cloth has a thickness of from 5 µm to 10 mm.

Inventors: KARIYAZONO YOSHIHISA, MIMURA HIROYUKI, MIYAJIMA YOSHIHIRO, SASAKI TOSHIYUKI, SATO HIROTAKA, YOSHIZAKI NOBUKI, KARIYAZONO, YOSHIHISA, MIMURA, HIROYUKI, MIYAJIMA, YOSHIHIRO, SASAKI, TOSHIYUKI, SATO, HIROTAKA, YOSHIZAKI, NOBUKI, KARIYAZONO, YOSHIHISA NIPPON STEEL CORP., MIMURA, HIROYUKI NIPPON STEEL CORP., MIYAJIMA, YOSHIHIRO NIPPON STEEL CORP., SASAKI, TOSHIYUKI NIPPON STEEL CORP., SATO, HIROTAKA NIPPON STEEL CORP.KIMITSU WORKS, YOSHIZAKI, NOBUKI NIPPON STEEL CORP.KIMITSU WORKS
Applicants: NIPPON STEEL CORP, NIPPON STEEL CORPORATION
Application year: 1994
Main classification: B32B15
IPC codes: B32B15, B32B27, F16L57